1. Field of the Invention
The invention relates to the general field of Integrated Circuit (IC) devices and fabrication methods, and more particularly to multilayer or Three Dimensional Integrated Circuit (3D-IC) devices
2. Discussion of Background Art
Over the past 40 years, one has seen a dramatic increase in functionality and performance of Integrated Circuits (ICs). This has largely been due to the phenomenon of “scaling” i.e. component sizes within ICs have been reduced (“scaled”) with every successive generation of technology. There are two main classes of components in Complementary Metal Oxide Semiconductor (CMOS) ICs, namely transistors and wires. With “scaling”, transistor performance and density typically improve and this has contributed to the previously-mentioned increases in IC performance and functionality. However, wires (interconnects) that connect together transistors degrade in performance with “scaling”. The situation today may be that wires dominate performance, functionality and power consumption of ICs.
3D stacking of semiconductor chips may be one avenue to tackle issues with wires. By arranging transistors in 3 dimensions instead of 2 dimensions (as was the case in the 1990s), one can place transistors in ICs closer to each other. This reduces wire lengths and keeps wiring delay low. However, there are many barriers to practical implementation of 3D stacked chips. These include:                Constructing transistors in ICs typically require high temperatures (higher than ˜700° C.) while wiring levels are constructed at low temperatures (lower than ˜400° C.). Copper or Aluminum wiring levels, in fact, can get damaged when exposed to temperatures higher than ˜400° C. If one would like to arrange transistors in 3 dimensions along with wires, it has the challenge described below. For example, let us consider a 2 layer stack of transistors and wires i.e. Bottom Transistor Layer, above it Bottom Wiring Layer, above it Top Transistor Layer and above it Top Wiring Layer. When the Top Transistor Layer may be constructed using Temperatures higher than 700° C., it can damage the Bottom Wiring Layer.        Due to the above mentioned problem with forming transistor layers above wiring layers at temperatures lower than 400° C., the semiconductor industry has largely explored alternative architectures for 3D stacking In these alternative architectures, Bottom Transistor Layers, Bottom Wiring Layers and Contacts to the Top Layer are constructed on one silicon wafer. Top Transistor Layers, Top Wiring Layers and Contacts to the Bottom Layer are constructed on another silicon wafer. These two wafers are bonded to each other and contacts are aligned, bonded and connected to each other as well. Unfortunately, the size of Contacts to the other Layer may be large and the number of these Contacts may be small. In fact, prototypes of 3D stacked chips today utilize as few as 10,000 connections between two layers, compared to billions of connections within a layer. This low connectivity between layers may be because of two reasons: (i) Landing pad size needs to be relatively large due to alignment issues during wafer bonding. These could be due to many reasons, including bowing of wafers to be bonded to each other, thermal expansion differences between the two wafers, and lithographic or placement misalignment. This misalignment between two wafers limits the minimum contact landing pad area for electrical connection between two layers; (ii) The contact size needs to be relatively large. Forming contacts to another stacked wafer typically involves having a Through-Silicon Via (TSV) on a chip. Etching deep holes in silicon with small lateral dimensions and filling them with metal to form TSVs may be not easy. This places a restriction on lateral dimensions of TSVs, which in turn impacts TSV density and contact density to another stacked layer. Therefore, connectivity between two wafers may be limited.        
It may be highly desirable to circumvent these issues and build 3D stacked semiconductor chips with a high-density of connections between layers. To achieve this goal, it may be sufficient that one of three requirements must be met: (1) A technology to construct high-performance transistors with processing temperatures below ˜400° C.; (2) A technology where standard transistors are fabricated in a pattern, which allows for high density connectivity despite the misalignment between the two bonded wafers; and (3) A chip architecture where process temperature increase beyond 400° C. for the transistors in the top layer does not degrade the characteristics or reliability of the bottom transistors and wiring appreciably. This patent application describes approaches to address options (1), (2) and (3) in the detailed description section. In the rest of this section, background art that has previously tried to address options (1), (2) and (3) will be described.
U.S. Pat. No. 7,052,941 from Sang-Yun Lee (“S-Y Lee”) describes methods to construct vertical transistors above wiring layers at less than 400° C. In these single crystal Si transistors, current flow in the transistor's channel region may be in the vertical direction. Unfortunately, however, almost all semiconductor devices in the market today (logic, DRAM, flash memory) utilize horizontal (or planar) transistors due to their many advantages, and it may be difficult to convince the industry to move to vertical transistor technology.
A paper from IBM at the Intl. Electron Devices Meeting in 2005 describes a method to construct transistors for the top stacked layer of a 2 chip 3D stack on a separate wafer. This paper is “Enabling SOI-Based Assembly Technology for Three-Dimensional (3D) Integrated Circuits (ICs),” IEDM Tech. Digest, p. 363 (2005) by A. W. Topol, D.C. La Tulipe, L. Shi, et al. (“Topol”). A process flow may be utilized to transfer this top transistor layer atop the bottom wiring and transistor layers at temperatures less than 400° C. Unfortunately, since transistors are fully formed prior to bonding, this scheme suffers from misalignment issues. While Topol describes techniques to reduce misalignment errors in the above paper, the techniques of Topol still suffer from misalignment errors that limit contact dimensions between two chips in the stack to >130 nm.
The textbook “Integrated Interconnect Technologies for 3D Nanoelectronic Systems” by Bakir and Meindl (“Bakir”) describes a 3D stacked DRAM concept with horizontal (i.e. planar) transistors. Silicon for stacked transistors may be produced using selective epitaxy technology or laser recrystallization. Unfortunately, however, these technologies have higher defect density compared to standard single crystal silicon. This higher defect density degrades transistor performance.
In the NAND flash memory industry, several organizations have attempted to construct 3D stacked memory. These attempts predominantly use transistors constructed with poly-Si or selective epi technology as well as charge-trap concepts. References that describe these attempts to 3D stacked memory include “Integrated Interconnect Technologies for 3D Nanoelectronic Systems”, Artech House, 2009 by Bakir and Meindl (“Bakir”), “Bit Cost Scalable Technology with Punch and Plug Process for Ultra High Density Flash Memory”, Symp. VLSI Technology Tech. Dig. pp. 14-15, 2007 by H. Tanaka, M. Kido, K. Yahashi, et al. (“Tanaka”), “A Highly Scalable 8-Layer 3D Vertical-Gate (VG) TFT NAND Flash Using Junction-Free Buried Channel BE-SONOS Device,” Symposium on VLSI Technology, 2010 by W. Kim, S. Choi, et al. (“W. Kim”), “A Highly Scalable 8-Layer 3D Vertical-Gate (VG) TFT NAND Flash Using Junction-Free Buried Channel BE-SONOS Device,” Symposium on VLSI Technology, 2010 by Hang-Ting Lue, et al. (“Lue”) and “Sub-50 nm Dual-Gate Thin-Film Transistors for Monolithic 3-D Flash”, IEEE Trans. Elect. Dev., vol. 56, pp. 2703-2710, November 2009 by A. J. Walker (“Walker”). An architecture and technology that utilizes single crystal Silicon using epi growth is described in “A Stacked SONOS Technology, Up to 4 Levels and 6 nm Crystalline Nanowires, with Gate-All-Around or Independent Gates (ΦFlash), Suitable for Full 3D Integration”, International Electron Devices Meeting, 2009 by A. Hubert, et al (“Hubert”). However, the approach described by Hubert has some challenges including the use of difficult-to-manufacture nanowire transistors, higher defect densities due to formation of Si and SiGe layers atop each other, high temperature processing for long times, and difficult manufacturing.
It is clear based on the background art mentioned above that invention of novel technologies for 3D stacked chips will be useful.
Three dimensional integrated circuits are known in the art, though the field may be in its infancy with a dearth of commercial products. Many manufacturers sell multiple standard two dimensional integrated circuit (2DIC) devices in a single package known as a Multi-Chip Modules (MCM) or Multi-Chip Packages (MCP). Often these 2DICs are laid out horizontally in a single layer, like the Core 2 Quad microprocessor MCMs available from Intel Corporation of Santa Clara, Calif. In other products, the standard 2DICs are stacked vertically in the same MCP like in many of the moviNAND flash memory devices available from Samsung Electronics of Seoul, South Korea like the illustration shown in FIG. 81C. None of these products are true 3DICs.
Devices where multiple layers of silicon or some other semiconductor (where each layer comprises active devices and local interconnect like a standard 2DIC) are bonded together with Through Silicon Via (TSV) technology to form a true 3D IC have been reported in the literature in the form of abstract analysis of such structures as well as devices constructed doing basic research and development in this area. FIG. 81A illustrates an example in which Through Silicon Vias are constructed continuing vertically through all the layers creating a global interlayer connection. FIG. 81B provides an illustration of a 3D IC system in which a Through Silicon Via 8104 may be placed at the same relative location on the top and bottom of all the 3D IC layers creating a standard vertical interface between the layers.
Constructing future 3DICs may require new architectures and new ways of thinking In particular, yield and reliability of extremely complex three dimensional systems will have to be addressed, particularly given the yield and reliability difficulties encountered in complex Application Specific Integrated Circuits (ASIC) built in recent deep submicron process generations.
Fortunately, current testing techniques will likely prove applicable to 3D IC manufacturing, though they will be applied in very different ways. FIG. 100 illustrates a prior art set scan architecture in a 2D IC ASIC 10000. The ASIC functionality may be present in logic clouds 10020, 10022, 10024 and 10026 which are interspersed with sequential cells like, for example, pluralities of flip flops indicated at 10012, 10014 and 10016. The ASIC 10000 also has input pads 10030 and output pads 10040. The flip flops are typically provided with circuitry to allow them to function as a shift register in a test mode. In FIG. 100 the flip flops form a scan register chain where pluralities of flip flops 10012, 10014 and 10016 are coupled together in series with Scan Test Controller 10010. One scan chain may be shown in FIG. 100, but in a practical design comprising millions of flip flops many sub-chains will be used.
In the test architecture of FIG. 100, test vectors are shifted into the scan chain in a test mode. Then the part may be placed into operating mode for one or more clock cycles, after which the contents of the flip flops are shifted out and compared with the expected results. This provides an excellent way to isolate errors and diagnose problems, though the number of test vectors in a practical design can be very large and an external tester may be often required.
FIG. 101 shows a prior art boundary scan architecture in exemplary ASIC 10100. The part functionality may be shown in logic function block 10110. The part also has a variety of input/output cells 10120, each comprising a bond pad 10122, an input buffer 10124, and a tri-state output buffer 10126. Boundary Scan Register Chains 10132 and 10134 are shown coupled in series with Scan Test Control block 10130. This architecture operates in a similar manner as the set scan architecture of FIG. 100. Test vectors are shifted in, the part may be clocked, and the results are then shifted out to compare with expected results. Typically, set scan and boundary scan are used together in the same ASIC to provide complete test coverage.
FIG. 102 shows a prior art Built-In Self Test (BIST) architecture for testing a logic block 10200 which comprises a core block function 10210 (what is being tested), inputs 10212, outputs 10214, a BIST Controller 10220, an input Linear Feedback Shift Register (LFSR) 10222, and an output Cyclical Redundancy Check (CRC) circuit 10224. Under control of BIST Controller 10220, LFSR 10222 and CRC 10224 are seeded (set to a known starting value), the logic block 10200 may be clocked a predetermined number of times with LFSR 10222 presenting pseudo-random test vectors to the inputs of Block Function 10210 and CRC 10224 monitoring the outputs of Block Function 10210. After the predetermined number of clocks, the contents of CRC 10224 are compared to the expected value (or “signature”). If the signature matches, logic block 10200 passes the test and may be deemed good. This sort of testing may be good for fast “go” or “no go” testing as it may be self-contained to the block being tested and does not require storing a large number of test vectors or use of an external tester. BIST, set scan, and boundary scan techniques are often combined in complementary ways on the same ASIC. A detailed discussion of the theory of LSFRs and CRCs can be found in Digital Systems Testing and Testable Design, by Abramovici, Breuer and Friedman, Computer Science Press, 1990, pp 432-447.
Another prior art technique that may be applicable to the yield and reliability of 3DICs is Triple Modular Redundancy. This may be a technique where the circuitry may be instantiated in a design in triplicate and the results are compared. Because two or three of the circuit outputs are always assumed in agreement (as may be the case assuming single error and binary signals) voting circuitry (or majority-of-three or MAJ3) takes that as the result. While primarily a technique used for noise suppression in high reliability or radiation tolerant systems in military, aerospace and space applications, it also can be used as a way of masking errors in faulty circuits since if any two of three replicated circuits are functional the system will behave as if it may be fully functional. A discussion of the radiation tolerant aspects of Triple Modular Redundancy systems, Single Event Effects (SEE), Single Event Upsets (SEU) and Single Event Transients (SET) can be found in U.S. Patent Application Publication 2009/0204933 to Rezgui (“Rezgui”).
Over the past 40 years, there has been a dramatic increase in functionality and performance of Integrated Circuits (ICs). This has largely been due to the phenomenon of “scaling”; i.e., component sizes within ICs have been reduced (“scaled”) with every successive generation of technology. There are two main classes of components in Complementary Metal Oxide Semiconductor (CMOS) ICs, namely transistors and wires. With “scaling”, transistor performance and density typically improve and this has contributed to the previously-mentioned increases in IC performance and functionality. However, wires (interconnects) that connect together transistors degrade in performance with “scaling”. The situation today may be that wires dominate performance, functionality and power consumption of ICs.
3D stacking of semiconductor devices or chips may be one avenue to tackle the issues with wires. By arranging transistors in 3 dimensions instead of 2 dimensions (as was the case in the 1990s), the transistors in ICs can be placed closer to each other. This reduces wire lengths and keeps wiring delay low.
There are many techniques to construct 3D stacked integrated circuits or chips including:
Through-silicon via (TSV) technology: Multiple layers of transistors (with or without wiring levels) can be constructed separately. Following this, they can be bonded to each other and connected to each other with through-silicon vias (TSVs).
Monolithic 3D technology: With this approach, multiple layers of transistors and wires can be monolithically constructed. Some monolithic 3D approaches are described in pending U.S. patent application Ser. No. 12/900,379 and U.S. patent application Ser. No. 12/904,119.
Irrespective of the technique used to construct 3D stacked integrated circuits or chips, heat removal may be a serious issue for this technology. For example, when a layer of circuits with power density P may be stacked atop another layer with power density P, the net power density may be 2 P. Removing the heat produced due to this power density may be a significant challenge. In addition, many heat producing regions in 3D stacked integrated circuits or chips have a high thermal resistance to the heat sink, and this makes heat removal even more difficult.
Several solutions have been proposed to tackle this issue of heat removal in 3D stacked integrated circuits and chips. These are described in the following paragraphs.
Many publications have suggested passing liquid coolant through multiple device layers of a 3D-IC to remove heat. This is described in “Microchannel Cooled 3D Integrated Systems”, Proc. Intl. Interconnect Technology Conference, 2008 by D.C. Sekar, et al and “Forced Convective Interlayer Cooling in Vertically Integrated Packages,” Proc. Intersoc. Conference on Thermal Management (ITHERM), 2008 by T. Brunschweiler, et al.
Thermal vias have been suggested as techniques to transfer heat from stacked device layers to the heat sink. Use of power and ground vias for thermal conduction in 3D-ICs has also been suggested. These techniques are described in “Allocating Power Ground Vias in 3D ICs for Simultaneous Power and Thermal Integrity” ACM Transactions on Design Automation of Electronic Systems (TODAES), May 2009 by Hao Yu, Joanna Ho and Lei He.
Other techniques to remove heat from 3D Integrated Circuits and Chips will be beneficial.